104 



Proceedings of the Royal Irish Academy. 



exposures in tributary streams which join the Little Eiver in the 

 neighbourhood of the Slate Quarry ; and in neither of them is the 

 actual passage upward or downward very clearly shown. 



The Lower Tirnaskea beds seem to be a direct continuation of 

 the highest flaggy member of the Killey Bridge beds ; but the little 

 anticlines which cause them to be exposed in the Tirnaskea stream 

 are only sufficiently dissected to allow a very little of these to be 

 seen ; while in the brook south of Bardahessiagh this portion of the 

 section is completely covered by rain-wash and drift. The lowest 

 Tirnaskea beds of the Tirnaskea stream are more gritty than any of 

 the Killey Bridge beds, and from the fact that they occur in beds 

 with goodcuboidal jointing, often about a foot thick, they are readily 

 recognizable. They are always tough and very ' blocky,' and though 

 they probably contain many fossils, these fossils are extremely difficult 

 to extract. The cementing material is some rhombohedral carbonate, 

 which is far from brittle, and the rock, whether weathered or fresh, 

 seems always to crush, rather than to break, under the hammer. 



Under the microscope, the rock is remarkable for the large propor- 

 tion of perfectly fresh oligoclase and andesine which it contains. Its 

 quartz grains, like those of the Bardahessiagh beds, are sharp and 

 angular, while, unlike the beds of the lower series, the mica, if 

 present, is only in the pasty ground-mass. A few grains of tolerably 

 fresh hornblende and a little brightly polarizing epidote are also 

 present among the well-sutured grains of the calcareous cementing 

 ground-mass. The well-known Ashgillian Trilobite Fhacops 

 mucronatus is represented by five large specimens with well-developed 

 eyes ; and a few specimens of the Hartfell Graptolites Bicellograptus 

 compla7iatus var., with Diplograptus truncatus, were obtained in 

 certain of the lenticular streaks of shale occurring within the 

 massive grit. 



The Upper Tirnaskea Beds. 



There is a very sudden transition from the Lower Tirnaskea grits 

 to the smooth-banded mudstones and shales of the Upper Tirnaskea 

 beds. These are only exposed for a very few feet, but the highest 

 zone fossil of the Hartfell Shales, Bicellograptus anceps^ was found in 

 the laminae of black or purple shale which form partings in the 

 green or grey mudstones at intervals of an inch or two. With it 

 also occurs ^glina rediviva, that much-discussed recurrent Trilobite 

 of Barrande's Bohemian colonies,'' which is here recognized for 

 the first time in Ireland. 



